There is provided a substrate for mounting electronic elements, including: a frame-shaped first wiring substrate with a first through hole formed in an interior portion thereof; a flat plate-shaped or frame-shaped second wiring substrate arranged to overlap with a bottom surface of the first wiring substrate and electrically connected thereto; and a sheet-shaped metal sheet arranged to overlap with a bottom surface of the second wiring substrate such that the second wiring substrate is sandwiched between the first wiring substrate and the metal sheet, an area inside the frame-shaped first wiring substrate or areas inside the frame-shaped first wiring substrate and second wiring substrate being electronic element mounting areas, and the second wiring substrate having an elastic modulus lower than those of the first wiring substrate and the metal sheet.